1. Field of the Invention
The present invention relates to a solid state imaging device, more particularly to a technology for correcting level variations in output signals from a solid state imaging device using an X-Y address type solid state imaging element for sequentially reading out pixel signals from pixels by X-Y addressing.
2. Description of the Related Art
A CCD (Charge Coupled Device) sensor and a CMOS (Complementary Metal Oxide Semiconductor) sensor are widely known as solid state imaging elements to be used for imaging devices such as a video camera and a digital camera. Among them, recently, the CMOS sensor has tremendously been improved in sensitivity. However, this causes a problem that when correcting or significantly reducing level variations in output signals from an imaging device using a CMOS sensor, a conventional correction method to adjust the exposure time of an electronic shutter to the period of the field or frame frequency of the CMOS sensor may not be able to perform the correction. This will be described in detail below.
Level variation of output signals is a phenomenon that occurs when an imaging device is used for imaging under illumination of a discharge type light source using an AC power supply such as a fluorescent lamp or a mercury lamp. When an X-Y address type solid state imaging element such as a CMOS sensor, in which each exposure is made for each pixel (pixel by pixel) or for each scan line (scan line by scan line), is used for imaging under the discharge type illumination, the timing for accumulating or storing charge varies for each pixel or each scan line. Accordingly, when an electronic shutter is released at a high speed, the shutter time may be shorter than the period of energy variation of the discharge type illumination, which causes each signal output from each pixel or scan line to vary in level or amplitude. This causes output signals varying in level or amplitude for each scan line, whereby so-called horizontal stripe noise occurs in the resultant image data from the imaging device. This is the phenomena called level variation.
If the discharge type illumination has a low luminance (intensity), it is possible to correct the level variation by selecting the exposure time of the electronic shutter to be equal to the period, or an integer multiple of the period, of the field frequency period) or the frame frequency of the CMOS sensor. However, if the illumination has a high luminance, the exposure time of the electronic shutter of the CMOS sensor may be required to be shorter than the period of the field or frame frequency of the CMOS sensor, preventing the exposure time from coinciding with the field or frame frequency. This will be described in more detail below.
Reference is made to FIG. 12, which is a timing chart of signals in a light source and a CMOS sensor according to a conventional solid state imaging device, where FIG. 12(a) is a graph showing a frequency of the light source or illumination, and FIG. 12(b) is a graph showing a period of energy variation of the light source or illumination. On the other hand, each of FIG. 12(c) and FIG. 12(e) is a chart showing an exposure time for each scan line in sequential exposure for individual scan lines, while each of FIG. 12(d) and FIG. 12(f) is a graph showing a waveform of output signals from the individual scan lines, representing levels or amplitudes (i.e. amounts of energy) of the output signals in sequential fields (or frames).
When the exposure time is Δt1 as shown in FIG. 12(c) which is equal to, or 1 multiple of, the period of energy variation as shown in FIG. 12(b), the levels or amplitudes of the output signals from the individual scan lines are constant or uniform as shown in FIG. 12(d). However, when the exposure time is Δt2 as shown in FIG. 12(e) which is shorter than the period of energy variation shown in FIG. 12(b) (for example, when the electronic shutter is released at a high speed), the amounts of energy of the light source accumulated for individual scan lines vary depending on the timing of exposure. As a result, the levels of the output signals from the individual scan lines vary as shown in FIG. 12(f). If such signals with the level variation are output from an imaging device, resultant image data from the imaging device generates horizontal stripe noise such that bright horizontal stripes and dark horizontal stripes appear in an alternating sequence in the scan lines.
In summary, the conventional correction method for a solid state imaging device using a CMOS sensor is to make the period of energy variation of the illumination equal to, or 1 multiple of, the field or frame frequency (rate) or the exposure time of an electronic shutter. However, the conventional correction method has a limitation in the ability to correct the level variation of output signals if the CMOS sensor has improved sensitivity to cause short exposure time as described above. As a result, the level variation of the output signals occurs which cannot be corrected or significantly reduced by the conventional correction method, causing low quality of images taken by the imaging device.
In contrast to the CMOS sensor, the problem described above does not occur in a CCD sensor due to its technical characteristics. This can be understood from FIG. 13, which is a timing chart of signals in a light source and a CMOS sensor according to a conventional solid state imaging device, where FIG. 13(a) is a graph showing a frequency of the light source (illumination), and FIG. 13(b) is a graph showing a period of energy variation of the light source. On the other hand, each of FIG. 13(c) and FIG. 13(e) is a chart showing an exposure time for each scan line in sequential exposure for individual scan lines, while each of FIG. 13(d) and FIG. 13(f) is a graph showing a waveform of output signals from the individual scan lines, representing levels or amplitudes (i.e. amounts of energy) of the output signals in sequential fields (or frames).
In the case of a charge transfer type solid state imaging device such as a CCD sensor, the accumulation of signal charges for all pixels starts at the same time, and the signal charges are read out from the pixels simultaneously (refer to FIG. 13). Accordingly, the accumulation time of signal charge (exposure time) is constant for all the pixels, so that the level variation among individual scan lines (inter-scan line variation) does not occur. However, even in the CCD sensor, inter-field or inter-frame level variation (flicker) may occur if the period of energy variation of the discharge type illumination is different from the field frequency or the frame frequency of output signals as in the case of FIG. 13(b) and FIG. 13(c).
A possible method for correcting the level variation (flicker) is to make the exposure time of the electronic shutter equal to the period, or a multiple of the period, of the energy variation of the discharge type illumination as shown in FIG. 13(e), thereby preventing flicker between fields or frames (inter-field or inter-frame flicker) from occurring. Furthermore, in the case of the CCD sensor, even if the exposure time of the electronic shutter cannot be made equal to the period, or a multiple of the period, of the field or frame frequency, or even if the illumination has a high luminance, it is possible to correct or significantly reduce flicker by varying the gain (amplification factor) of the output signals among periods of the field or frame frequency (inter-period variation).
On the other hand, in the case of the CMOS sensor, which is an X-Y address type solid state imaging element, the levels or amplitudes of the output signals vary among individual scan lines or individual pixels (inter-scan line or inter-pixel variation). This is regardless of the frequency of the illumination, whether 50 Hz or 60 Hz. Accordingly, the method for correcting the level variation and flicker in the CCD sensor cannot be used in the same manner for the CMOS sensor. Further, the level variation of the CMOS sensor increases with the increase of the luminance, so that the CMOS sensor has been considered unsuitable for use as a sensor in a solid state imaging device such as a digital camera which requires high sensitivity. Thus, the mainstream technology has been to use, as a solid state imaging element, the CCD sensor that enables correction of the level variation and flicker relatively easily.
Several technologies have been proposed to solve the problem occurring in the CMOS sensor. For example, Japanese Laid-open Patent Publication 2003-032551 discloses a solid state imaging element and a method for driving the same as well as an imaging device using the same that can reduce flicker which occurs when a high speed electronic shutter is released for imaging under illumination of a discharge type light source. According to this patent publication, at least two divided exposure times are set for each pixel in a CMOS sensor, in which the exposure start time of each divided exposure time is determined on the basis of the period of energy variation of the illumination.
For example, if the frequency of the illumination is 50 Hz, and if two divided exposure times are used, a period of energy variation of the illumination of 10 msec, which is one period here, is divided into two 5 ms durations. A set of two exposures, each of which has an exposure time of half of normal exposure time, is made starting from the start of each of the two 5 ms durations. In other words, a first exposure is made at normal timing or normal start time (with exposure time which is half of the normal exposure time). Thereafter, a second exposure is made at a point delayed by 5 ms from the normal start time (also with half of the normal exposure time). In the CMOS sensor, charges are generated by the two exposures, and are integrated and output as output signals so as to correct or significantly reduce the level variation and flicker.
This correction method of Japanese Laid-open Patent Publication 2003-032551 is disadvantageous in that the period of energy variation of the illumination is not related to the length of the exposure time. If the exposure time is an integer multiple of the illumination frequency, the amount of the integrated signal charges to be output is constant or uniform, whereby sufficient correction can be achieved. However, if the exposure time is not an integer multiple of the illumination frequency, the amount of the integrated signal charges to be output is not constant. Accordingly, although it may be possible to reduce the level variation and flicker to some extent, it is not possible to correct or eliminate them.
Another technology is proposed in Japanese Laid-open Patent Publication Hei 6-253216. This patent publication discloses an interline CCD sensor, and a method for driving the same so as to adjust the exposure time. The interline CCD sensor is formed such that vertical registers are provided between sensor cells (e.g. photodiodes) for generating and accumulating signal charges, in which the accumulated signal charges are transferred to the vertical registers, and are read out by providing the vertical registers with vertical transfer clock signals.
The method for driving the CCD sensor according to this patent publication has an imaging time which is stopped by applying, to the vertical registers, a voltage to cause a transfer of signal charges from the sensor cells. The imaging time is divided into N (N being integer, N≧2) time periods each of which is divided into a drain period (invalid exposure time), in which signal charges accumulated in the sensor cells are drained, and an accumulation period (valid exposure time) in which signal charges are accumulated in the sensor cells. A feature of this method for driving the CCD sensor is that the invalid exposure times are controlled so as to control the sensitivity of the CCD sensor, while the signal charges transferred to the vertical registers are read out by providing the vertical registers with the vertical transfer clock signals.
However, a problem arises in the driving method of Japanese Laid-open Patent Publication Hei 6-253216 if it is used with a CMOS sensor. More specifically, the driving method of this patent publication can be used only with a charge transfer type solid state imaging element such as a CCD sensor, which has a mechanism of selective transfer of charges, either to transfer signal charges accumulated in the sensor cells to the vertical registers, or to drain the accumulated charges to or toward the substrate (such as a readout gate and a charge drain mechanism). For this reason, the driving method of this patent publication cannot be used for an X-Y address type solid state imaging element such as a CMOS sensor, which outputs signals from each scan line, scan line-by-scan line. This means that this driving method cannot be used by itself, and requires some improvement or combined use of another function (e.g. use of a global shutter), so as to be usable for the CMOS sensor.
A technology to solve the problem occurring in the CMOS sensor is proposed in Japanese Laid-open Patent Publication 2005-27137. This patent publication discloses an imaging device using e.g. a CMOS sensor that can reduce flicker in imaging under illumination with energy variation, and reduce distortion when imaging a moving object. When used for imaging under discharge type illumination with a predetermined illumination frequency, the imaging device of this patent publication sets a predetermined continuous exposure time in each vertical interval corresponding to the predetermined illumination frequency, and obtains a base divisor determined by the phase difference between the set exposure time and the vertical sync frequency. The exposure time is divided by an integer multiple of the base divisor to determine division points in the exposure time. Taking the thus determined division points as starting points of exposure and readout, the imaging device performs exposure and signal readout simultaneously or substantially simultaneously at the division points. The thus read out signals are integrated for each pixel, pixel-by-pixel, so as to generate an image signal.
In the imaging device of this Japanese Laid-open Patent Publication 2005-27137, the area of an imaging element is divided, and accumulated charges are read out from the divided areas by shifting the readout timing for each of the respective divided areas. However, a problem of complexity in device configuration arises in the imaging device of this patent publication. More specifically, this imaging device requires external memories for integrating the accumulated charges in the respective divided areas, so that the device configuration requires some addition or modification. Furthermore, in order to increase the accuracy of correction of the flicker, it is necessary to increase the number of divided areas and the number of corresponding external memories. This causes the imaging device and its process to inevitably be complex and costly (e.g. cost increase due to the increase of external memories).